In March 2016, NASA selected two proposals for new Earth science investigations that will put new instruments in low-Earth orbit to track harmful particulate air pollutants and study the development and lifecycles over much of TCs (Tropical Cyclones).The measurements will provide nearly all-weather observations of 3D temperature and humidity, as well as cloud ice, precipitation horizontal structure and instantaneous surface rain rates. These measurements and the increased temporal resolution provided by the CubeSat constellation, are needed to better understand the TC lifecycles and the environmental factors that affect the intensification of TCs. 1)2)3)

Observations of small atmospheric aerosols from the MAIA (Multi-Angle Imager for Aerosols) will be combined with health information to determine the toxicity of different particulate matter types in airborne pollutants over the world's major cities.

The TROPICS investigation will develop and launch a constellation of CubeSats to study the development of tropical cyclones through rapid-revisit sampling. William Blackwell of MIT/LL (Massachusetts Institute of Technology'/Lincoln Laboratory) in Lexington is the PI (Principal Investigator) of the TROPICS mission.

The TROPICS science program is directly relevant to three of the six NASA Earth Science Focus Areas: Weather, Water and Energy Cycle, and Climate Variability and Change. TROPICS addresses goals and objectives from the 2014 NASA Strategic Plan including advancing the understanding of Earth and developing technologies to improve the quality of life on our home planet (strategic goal 2) and advancing knowledge of Earth as a system to meet the challenges of environmental change and to improve life on our planet (objective 2.2).

The fundamental physical parameters required to address the TROPICS science objectives are 3D atmospheric temperature and humidity, storm intensity, and horizontal precipitation structure. These parameters have a long heritage of being derived from spaceborne PMW imagery and sounding channels (e.g., AMSU, ATMS, SSMIS). Practical considerations of antenna and instrument size and mass for a CubeSat system guide the selection of PMW (Passive MicroWave) channels for TROPICS.

Temperature and moisture profiles are retrievable from seven channels near 118 GHz and three near 183 GHz, respectively. Precipitation structure is obtained from a combination of 90 GHz, 206 GHz, and the temperature and moisture channels, with horizontal resolution matching that of the moisture data due to the high sensitivity to precipitation hydrometeors at 183 GHz. The 206-GHz channel will be sensitive to smaller ice particles than the 90-GHz channel and will generally produce a stronger signal. These observables link back to science requirements and to the primary sensor requirements (horizontal and vertical resolution and sensitivity).

The TROPICS constellation will consist of six 3U CubeSats, each about 30 cm long with a mass of 6 kg, that use scanning microwave radiometers to measure temperature, humidity, precipitation and cloud properties. The CubeSats will be launched into three separate orbital planes to enable the overall constellation to monitor changes in tropical cyclones as frequently as every 21 minutes.

The TROPICS team has previous experience developing CubeSats and analyzing satellite measurements of storms, and includes partnerships with NASA's Wallops Flight Facility in Wallops Island, Virginia, Goddard, several universities and NOAA (National Oceanic and Atmospheric Administration).

Figure 2: A constellation of identical 3U CubeSats provide sounding (left CubeSat has a temperature profile of a simulated TC (Tropical Cyclone) from a NWP (Numerical Weather Prediction) model and 12-channel radiometric imagery (center CubeSat has simulated radiances from NWP model and radiative transfer model and the near right CubeSat has a single channel radiance image of a TC) with 30-minute median revisit rate to meet most PATH requirements (image credit: MIT/LL, NASA, Ref. 5)

The spectrometer payload consists of a rotating passive RF antenna measuring spectral radiance as it rotates about the CubeSat velocity vector. The payload is based upon a similar design previously flown by MIT/LL on the MicroMAS-2 (Micro-sized Microwave Atmospheric Satellite-2)mission. However, a significant amount of redesign has been required to meet TROPICS performance and mission reliability requirements. The redesign includes:

A notional SV including the bus and payload is shown in Figure 5. The MicroMAS-2 bus does not have sufficient pointing accuracy or power generation capability to meet the TROPICS mission requirements. The TROPICS bus will match much of the functionality of the MicroMAS-2 bus, but will take advantage of recent advances in CubeSat bus technology and reliability.

BCT will build seven identical XACT-based 3U-class CubeSats for the TROPICS (Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats) mission. The satellites will be divided into three low-Earth orbital planes and will consist of a single high-performance radiometer payload hosted on each spacecraft bus. Each payload includes a BCT-designed motor as well as electronics to control the MIT/LL (Massachusetts Institute of Technology/Lincoln Laboratory) payload spin mechanism. 6)

• The first TROPICS application workshop was held at the University of Miami, Miami, Florida, USA on 8-10 May 2017.

• NASA selected the mission as the winning proposal for the Earth Venture1–Instrument (EVI-3) Announcement of Opportunity in 2016. William Blackwell of MIT/LL (Massachusetts Institute of Technology/Lincoln Laboratory) is the principal investigator for TROPICS. 7)

Launch: Launch readiness of the TROPICS mission is planned for late 2019.

Analyses have shown orbit lifetime of 9 years, well over the expected mission lifetime of one year and well in advance of the 25-year de-orbit requirement. This configuration yields 30-minute median revisit rates with average spatial resolution better than precipitation and all-weather temperature and humidity (PATH) requirements, all from a low-cost, low-risk CubeSat constellation platform.

Sensor complement (Two Radiometers)

TROPICS will fly two total power radiometers that measure 12 channels spanning approximately 90 to 206 GHz. The "WF-band" radiometer comprises eight channels from 90–119 GHz, and the "G-band" radiometer comprises four channels from 183–206 GHz. The radiometer block diagram is shown in Figure 6. The specific channel properties are shown in Table 2. The full-width at half maximum antenna beamwidths are achieved using an offset parabolic reflector illuminated with two electroformed feed horns that are physically separated, and the beams are combined and collocated using a polarizing wire grid diplexer. 8)

Figure 6: TROPICS radiometer block diagram (image credit: MIT/LL)

Channel

Center Frequency (GHz)

Bandwidth (MHz)

Beamwidth (º) Down/Cross

NEDT

Cal. Acc. (K)

1

91.655 ± 1.4

1000

3.00/3.17

0.95

2.0

2

114.50

1000

2.4/2.62

0.55

1.5

3

115.95

800

2.4/.2.62

0.60

1.5

4

116.65

600

2.4/.2.62

0.70

1.5

5

117.25

600

2.4/.2.62

0.70

1.5

6

117.80

500

2.4/.2.62

0.75

1.5

7

118.24

380

2.4/.2.62

0.85

1.5

8

118.58

300

2.4/.2.62

1.00

1.5

9

184.41

2000

1.5/1.87

0.60

1.0

10

186.51

2000

1.5/1.87

0.60

1.0

11

190.31

2000

1.5/1.87

0.60

1.0

12

204.8

2000

1.4/1.76

0.60

1.0

Table 2: Specification of the TROPICS radiometer channels

The antenna engineering model is shown in Figure 7. Beam efficiencies for the temperature and water vapor sounding channels are designed to exceed 95%. The simulated G-band antenna patterns are shown in Figure 8 and measured return loss of the G-band feedhorn is shown in Figure 9. Excellent performance is indicated in both the pattern and return loss data. Radiometer calibration is accomplished using weakly coupled noise diodes with known and stable noise output that are turned on and off against the cold space background. Satellite intercalibration is optimized using cross comparisons (9)) and daily calculated numerical model residuals (10)) to derive and implement any needed bias corrections. The W/F-band receiver assembly is shown in Figure 10. A custom SiGe MMIC was developed at UMass-Amherst to provide and RF amplifier, mixer, and IF preamplifier in a highly integrated package.

Figure 7: Fabricated engineering model of the TROPICS antenna assembly. The 90-120 GHz feedhorn is visible in the front of the photo, and the 180-205 GHz feedhorn can be seen through the wire grid (image credit: Thomas Keating, LTD)

The radiometer operates in an "integrate-while-scanning" mode that results in elongated footprints in the cross-track direction. The spatial resolution is thus reported as the geometric mean of the minor and major axes of the ellipse projected on the earth, also accounting for earth curvature. As the constellation of six satellites scans the earth, the footprints near the edge of the scan are revisited more often than the footprints near nadir. This is effect is quantified by calculating an "effective" spatial resolution that weights the spatial resolution of each footprint by the relative frequency by which it is revisited. The nadir, mean-across-scan, and effective spatial resolutions are shown in Table 3. The satellite pointing accuracy and sensor mounting requirements are set to ensure geolocation errors are smaller than approximately 10% of the footprint size.

Band

Nadir (km)

Scan Mean (km)

Effective Across Scan (km)

W (90 GHz)

29.6

42.9

50.7

F (118 GHz)

24.1

34.9

41.2

G (183 GHz)

16.1

23.3

27.5

G (205 GHz)

15.2

22.1

26.0

Table 3: TROPICS spatial resolution (shown in km) for W, F, and G-band channels are shown at nadir and averaged over the 81 footprints in the swath. Also shown is the "effective" spatial resolution that accounts for how often the footprints are revisited across the scan

The temperature weighting functions for all 12 TROPICS channels are shown in Figure 11. Channel passbands are designed to span altitudes from the surface up to 20 km for temperature and 10 km for water vapor. Multiple temperature channels probe the upper troposphere to observe tropical cyclone warm core anomalies.

Figure 10: The TROPICS W/F-band receiver assembly in development by UMass-Amherst. Shown from L–R are the coupled noise diode module, the RF preamplifier module, and the SiGe mixer/tripler/amplifier module (image credit: UMass, MIT/LL)

Figure 11: Weighting functions calculated at nadir incidence over a perfectly emissive surface for a standard tropical atmosphere for both a) temperature/imaging and b) water vapor/imaging channels (image credit: MIT/LL)

TROPICS summary: TROPICS will be the first demonstration that science payloads on low-cost CubeSats can push the frontiers of spaceborne monitoring of the Earth to enable system science and will fill gaps in our knowledge of the short time scale — hourly and less — evolution of tropical cyclones, where current capabilities are an order of magnitude slower. The TROPICS mission will implement a spaceborne earth observation mission designed to collect measurements over the tropical latitudes to observe the thermodynamics and precipitation structures of TCs over much of the storm lifecycle. The measurements will provide nearly all-weather observations of 3D temperature and humidity, as well as cloud ice and precipitation horizontal structure. These measurements and the increased temporal resolution provided by the CubeSat constellation, are needed to better understand the TC lifecycles and the environmental factors that affect the intensification of TCs (Ref. 3).

Ground Stations:

The TROPICS 3U CubeSats will interface with the KSAT-lite ground station network to allow for satellite command and control and downlink of bus and payload telemetry for each CubeSat in the constellation.

Data Processing: MIT/LL will interact with the mission operations provider to acquire the downlinked raw science data and format it into data products that can be shared with the data processing center at the University of Wisconsin. The data products will be made available to the data processing center via a secured connection. The data will be stored at MIT/LL in a SQL database on a MIT/LL computer system that includes disk redundancy and data backups. The entire mission data set will be stored at MIT/LL for the duration of the TROPICS project. The key elements of the TROPICS mission are shown in Figure 12.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).